Due to chemical reactions and the incomplete combustion of fuel in the combustion chamber of an internal combustion engine, the gases emitted from the exhaust system of the engine may contain a number of hazardous substances, which can lead to air pollution problems that may be detrimental to health and the environment. The main pollutants of concern in the gases emitted from the exhaust system of an internal combustion engine are hydrocarbons (HC), carbon monoxide (CO), oxides of nitrogen (NOx), and particulates (i.e. soot).
An ever-increasing number of vehicles has resulted in increasing air pollution, particularly in urban areas. Therefore, a series of ever-stricter emission standards have been imposed to mitigate these pollution problems.
In order to reduce the emissions of HC, CO, and NOx from gasoline-fuelled spark ignition engined vehicles, three-way catalytic converters have been widely implemented, such three-way catalytic devices incorporating a reduction catalyst, to reduce NOx to N2 and O2, and an oxidation catalyst, to oxidize CO to CO2 and HC to H2O and CO2. However, for optimal operation of a three-way catalyst, the vehicle engine has to be controlled to operate stoichiometrically, i.e. so that the amount of oxygen supplied to the combustion chamber corresponds to that required for complete combustion of the amount of fuel supplied. For gasoline, this corresponds to an air/fuel mass ratio of 14.7 parts air to 1 part fuel.
When there is more air, and hence oxygen, than required, then the system is said to be running lean, and the system is in oxidizing condition. In that case, the converter's two oxidizing reactions (oxidation of CO and HC) are favoured, at the expense of the reducing reaction. When there is excessive fuel, then the engine is running rich. The reduction of NO, is favoured, at the expense of CO and HC oxidation. If an engine could be held at the strict stoichiometric point for the fuel used, it is theoretically possible to reach 100% conversion efficiencies.
Unlike spark ignition gasoline-fuelled engines, diesel-fuelled compression-ignition engines are normally operated under lean conditions with excess air (λ>1). Thus, while an oxidation catalyst can be used to oxidize CO to CO2 and HC to H2O and CO2, excess oxygen in the exhaust gas due to the lean burn conditions normally prevents reduction of NOx to N2 and O2.
Selective catalytic reduction (SCR) can be used to reduce the NOx, wherein a gaseous or liquid reductant (most commonly ammonia or urea) is added to the exhaust gas stream and is adsorbed onto a catalyst. The reductant reacts with NOx in the exhaust gas to form H2O (water vapour) and N2 (nitrogen gas). However, SCR is very sensitive to fuel contaminants, operates in a limited (high) temperature window.
In diesel engine applications requiring oxidation of CO and HC as well as high NOx conversion, one currently uses a combination of a diesel oxidation catalyst (DOC) device, diesel particulate filter (DPF) and an SCR device with injection of urea (typically vaporized from aqueous solution, available e.g. as “Diesel Exhaust Fluid” or “AdBlue”) or ammonia. This is typically the case for applications on the US emission cycle and for heavy cars.
Once the SCR system reaches his operating temperature, NOx conversion efficiency higher than 95% is state of the art today.
The SCR catalyst is usually located at quite some distance away from the engine exhaust valves or the turbine-outlet. Typically, the DOC device and the particulate filter are located in a close-coupled position and the SCR catalyst is located underfloor. This is due to the fact that a minimum distance is required between the particulate filter outlet and the SCR catalyst inlet to accommodate the urea injector, the mixer element and some pipe length to allow the mixing of the urea or ammonia with the exhaust gas before the mixture of exhaust gas and urea enters the SCR catalyst.
An SCR catalyst becomes active around 220° C. Due to the long distance in the exhaust line it is, however, difficult to get the SCR system operational very early in the emission cycle.
In the past, one has applied special warm-up strategies in the engine management to bring the SCR catalyst as fast as possible to its operating temperature.
These warm-up strategies have a negative impact on fuel economy, and even with the best warm-up strategies, most applications have difficulties to achieve good NOx conversion efficiency during phases of heavy acceleration occurring shortly after a cold start or when the motor has not yet reached its temperature range for optimal operation. Tests following the FTP-75 cycle (Federal Test Procedure) have shown that with current state-of-the-art after-treatments systems, as much as about 50% of the total tailpipe NOx emissions are produced during the heavy accelerations occurring about 200 s after the beginnings of the “cold start phase” and the “hot start phase”, respectively.
Document DE 10 2005 022 420 A1 discloses an exhaust system for a Diesel engine with a three-way catalyst and an SCR device. The document suggests running the engine in stoichiometric mode during the warm-up phase. During stoichiometric operation, emissions are treated thanks to the three-way catalyst material.